1. Field of the Invention
The present invention relates to a liquid crystal display device. More specifically, the present invention relates to a liquid crystal display device having a liquid alignment film formed by a photo-alignment method.
2. Description of the Related Art
In general, a liquid crystal display device performs a display using a change in optical characteristics of a liquid crystal layer caused by applying an electric field to liquid crystal molecules in a liquid crystal layer sandwiched between a pair of substrates to change the alignment of the liquid crystal molecules.
In a pair of substrates, a liquid crystal alignment film typically made of polyimide, polyamic acid, or the like is provided on an interface between a liquid crystal layer and a substrate and plays a role of aligning liquid crystal molecules in the liquid crystal layer in a predetermined direction.
As an industrial method of providing an organic coating film formed of polyimide or the like with a function of aligning liquid crystal molecules (hereinafter, referred to as “alignment capability”), a rubbing alignment process is widely known.
The rubbing alignment process is a method of rubbing the surface of an organic coating film formed on the surface of a substrate with a cloth made of nylon, polyester, or the like in a predetermined direction. This provides the surface of the organic coating film with alignment capability in the rubbed direction.
The rubbing alignment treatment is an industrially effective method because the treatment can provide the surface of an organic coating film with uniform alignment capability relatively easily and in addition, is excellent in productivity. However, along with the recent enlargement of liquid crystal display devices, problems of the rubbing alignment treatment have come to be pointed out.
As described above, the rubbing alignment treatment includes the step of allowing an organic coating film and a cloth to rub against each other physically. Therefore, unwanted shavings (scrapings) may be generated on the surface of the liquid crystal alignment film thus formed. The shavings cause display defects in a liquid crystal display device. The amount of shavings increases along with the enlargement of a liquid crystal display device, and hence there is apprehension that a defect occurrence ratio may increase. Consequently, there is a demand for the establishment of a treatment method that can replace the rubbing alignment treatment.
As an alignment treatment method replacing the rubbing alignment treatment, a photo-alignment process is known. The photo-alignment process is a method of providing a surface of an organic coating film with alignment capability by irradiating the surface of the organic coating film formed on the surface of a substrate with substantially linearly polarized UV rays (see, for example, U.S. Pat. No. 3,937,432).